Investigations will be directed at elucidating the biochemical nature of cell-cell interactions that influence development and differentiation in the nervous system. Reaggregate cultures will be prepared from specific brain regions of fetal and newborn mice. The differentiation of the cultures will be followed biochemically, morphologically and electrically. Cells from specific brain regions will be fractionated and used in various combinations to assess the role of specific cell types in brain reconstitution and development. The effects of various drugs, hormones and abnormal and excessive metabolites on in vitro differentiation will be determined.